1. Field of the Invention
The present invention relates to the bonding of copper composites to nonmetallic materials. In one of its more particular aspects this invention relates to the direct bonding of copper composites to ceramics.
2. Description of Related Art
In the design of electronic circuits, a principal consideration is the need to control temperature in order to improve reliability of the circuit components and to meet circuit performance requirements. In particular, in the case of power hybrids, which may have power densities exceeding 5 watts/in.sup.2, it is essential to provide a heat sink, or heat spreader, such as a metal layer, to remove a considerable proportion of the heat generated in the power hybrid by the active and passive devices. Similar considerations are applicable to other microcircuits such as standard hybrids and multi-chip modules.
Copper layers have found wide use as heat sinks. Typically, they have been bonded to ceramic members by soldering or brazing. A disadvantage of such bonding techniques is that they introduce an additional thermal interface between the heat sink and ceramic member. Voids may also occur during the soldering or brazing process which adversely affects thermal conductivity.
U.S. Pat. No. 3,766,634, in an attempt to eliminate such interfaces and voids, describes the direct bonding of copper and other metals to nonmetallic substrates. A copper-copper oxide eutectic is used to bond copper to an alumina substrate, for example.
U.S. Pat. No. 4,563,383 describes a direct bond copper ceramic substrate for use in high temperature thick film processing. The substrate consists of two outer ceramic layers and a central copper core formed of three layers bonded by a copper oxygen eutectic to the outer ceramic layers. By bonding ceramic to both sides of the copper core, stress symmetry is said to be provided.
One problem encountered in the provision of direct bond metal ceramic structures is that there is a marked temperature coefficient of expansion mismatch between the metal and the ceramic. For example, pure copper has a coefficient of expansion of 16.8 ppm/.degree.C. while alumina, beryllia and aluminum nitride, which are typical ceramic materials used in microcircuit packaging, have coefficients of expansion ranging from about 4.3 to 7.0 ppm/.degree.C. Because of this mismatch, the thickness of the copper layer is severely limited. For example, with a 0.025-inch thick ceramic substrate, the thickness of the copper layer is limited to approximately 0.012 inch, because thicker copper layers would result in considerable warping of the resulting structures.
Thicker copper layers, however, are desirable to attach the copper ceramic material to a base structure, since the ceramic is too brittle to allow attachment by screwing down to the base structure.
In an effort to avoid the adverse effects of the mismatch of coefficients of expansion of metals and ceramics, approximately equal amounts of copper have been provided on both the top and bottom sides of the ceramic substrate to balance out the stresses. The thickness of such copper layers can be approximately one-tenth to one-third the substrate thickness. For example, with a 0.025 inch thick substrate, each copper layer might be approximately 0.008 inch thick, which is too thin to provide sufficient rigidity for attachment to a base structure. It would be desirable to provide heat sinks for hybrids and multi-chip modules having sufficient rigidity to allow screwing down the heat sink to a base structure. It would also be desirable to avoid the need for using amounts of the metal layers on both sides of the ceramic substrate.
Accordingly, it is an object of this invention to provide a heat sink directly bonded to a ceramic member, which heat sink is of sufficient rigidity to be readily attached by screwing down to a base structure.
Another object of this invention is to provide metal ceramic structures in which the metal and ceramic have similar temperature coefficients of expansion.
A further object of this invention is to provide a process for bonding together a heat sink and a ceramic member without requiring soldering or brazing.
Other objects and advantages of this invention will become apparent in the course of the following detailed disclosure and description.